Spirillum, genus of spiral-shaped bacteria of the family Spirillaceae, aquatic except for one species (S. minorminus) that causes a type of rat-bite fever in man. The term spirillum is used generally for any of the corkscrew-like species.

Spirillum is microbiologically characterized as a gram-negative, motile helical cell with tufts of whiplike flagella at each end. The helix of the largest spirillum, S. volutans, is 5 to 8 μm (micrometres; 1 μm = 10-6 metre) across by 60 μm long.

S. minorminus, found in the blood of apparently healthy mice and rats, can be transmitted to other rodents and to monkeys and man.

The genera Aquaspirillum and Oceanospirillum have been proposed for the free-living aquatic forms.